Metal halide lamps and other gas-discharge lamps are commonly used in a number of venues such as sporting arenas and stadiums, plant nurseries, and industrial plants. Like other gas-discharge lamps, metal halide lamps produce light by passing an electric arc through a mixture of gases contained in a discharge vessel (e.g., argon, mercury, and metal halides). The argon is readily ionized, and enables striking the arc across the lamp electrodes when voltage is applied to the lamp. The heat generated by the arc in turn vaporizes the mercury and metal halides, which produces light as the temperature and pressure increases within the discharge vessel (also referred to as an arc tube or burner). The halides generally control the color and intensity of the light produced.
There are a number of conventional techniques for controlling light output of metal halide lamps and other gas-discharge lamps during essentially two scenarios: lamp run-up and hot relight. For instance, conventional techniques for controlling light output during gas-discharge lamp run-up include: optical feedback; predetermined power vs. time to be applied; voltage feedback, including estimation of lamp efficacy as a function of lamp voltage; and estimation of lamp efficacy as a function of total energy delivered to lamp. Techniques for controlling hot relight of a gas-discharge lamp include: tracking time since lamp shut-off to modify the predetermined power vs. time to be applied; and using voltage feedback.
There are a number of non-trivial and subtle issues associated with controlling light output of gas-discharge lamps.